The invention relates to an optical-fiber measuring device permitting the measurement of the variation of a parameter which produces non-reciprocal disturbances in a SAGNAC ring interferometer.
The SAGNAC interferometer and the physical phenomena which it utilizes are well known. In such an interferometer, a beam splitter or some other separating device divides an incident wave. The two oppositely propagating waves thus created propagate in opposite directions along a closed optical path, recombine and generate interferences which are dependent upon the phase shift of the waves in the course of their recombination.
Originally, the closed optical path of the SAGNAC interferometers was defined by mirrors. It is now known that it can be formed by a multi-turn coil of monomode optical fibre.
It is likewise known that certain physical phenomena are capable of producing disturbances, particularly phase shifts, which are non-reciprocal, on the oppositely propagating waves giving rise to a relative phase shift of these waves which modify their state of interference in the course of their recombination.
The measurement of this relative phase shift permits the quantification of the physical phenomenon which has given rise to it.
The principal physical phenomenon capable of creating these non-reciprocal disturbances is the SAGNAC effect produced by the rotation of the interferometer in relation to an axis perpendicular to the plane of its closed optical path. The FARADAY effect, or collinear magneto-optical effect, is likewise known as producing non-reciprocal effects of this type. This has, for example, been described in an article in the journal Optic Letters (Vol. 7, No. 4, Apr. 1982, pages 180-182) by K. BoHM. Under certain conditions, other effects may likewise produce a non-reciprocal phase shift.
On the other hand, the variations of numerous parameters representing the environment which frequently give rise to disturbances of the measurements have only reciprocal effects on the SAGNAC interferometer, do not disturb the relative phase shift between the oppositely propagating waves and therefore have no influence on the measurement of the parameter under investigation. This is so in the case of slow variations of temperature, of indices etc. . . which modify the optical path traversed by the waves, but modify it in a reciprocal manner.
Numerous projects have been undertaken for the purpose of improving the sensitivity and the precision of the measurements which can be made with such a measuring apparatus. It will be possible, for example, to consult the document GB 2 152 207 and the publication Electronics Letters (Vol. 19 No. 23, Nov. 1983 pages 997-999, an article by K. BoHM).
In particular, it has first of all been found that the response provided by the SAGNAC interferometer is of the form P=Po(1+cos. .DELTA..phi.) and thus that the sensitivity of this signal close to the phase difference .DELTA..phi.=0 is low. It has been proposed to introduce a phase difference modulation, squared with an amplitude of plus or minus .pi./2 for example, which displaces the operating point and produces a periodic signal, the amplitude of which is a sinusoidal function of the measured parameter and which it is therefore possible to use with a greater sensitivity and stability.
It was then shown that the precision of the measurement is improved by the use of a zero method which is likewise referred to as closed-loop operation. According to this method, a supplementary phase shift referred to as a feedback phase shift .DELTA..phi..sub.A is applied and serves to compensate the phase shift .DELTA..phi..sub.B produced by the measured parameter. The sum of these two phase shifts .DELTA..phi..sub.A and .DELTA..phi..sub.B is maintained at zero; this permits the interferometer to be operated with the maximum precision. The measurement is made by use of the signal required for the production of the feedback phase shift .DELTA..phi..sub.A. Thus, the measurement is stable and linear.
The control may be produced by generating phase progressions of a height .DELTA..phi..sub.A at each time .tau., .tau. being the propagation time in the coil, the phase modulator or modulators being placed at the ends of the coil.
The European Patent EP 0,168,292 describes such a measuring system. According to the device which it proposes, the signal produced by variation of the measured parameter produces a variation of the output signal of the detector. The amplitude of this variation is extracted by circuits for analog synchronous detection which, after analog processing by a DIP (differential, integral, proportional) filter, conventionally ensures the stability of the control loop. An analog-digital converter gives the digital value of the progression .DELTA..phi..sub.A which has been introduced in order to ensure the compensation, and there are added a control signal generator, the purpose of which is to formulate a digital ramp in steps, and finally a digital-analog converter which generates the drive signal returning from the phase modulator, on the basis of this ramp.
In seeking a maximum sensitivity and precision of the measurement, the devices of the prior art exhibit certain disadvantages:
An analog synchronous detection (also referred to as demodulation) generally involves a zero drift at the output which is reflected in an error in the measurement. PA1 The analog-digital converter required to construct this device must be able to process the compensation phase shift directly. It must include a number of bits linked to the maximum amplitude of the signal at its input; this leads, in practice, in order that it should not limit the precision of the measurement, to an analog-digital converter including a number of bits of the order of 18.